This invention relates to methods of controlling the condensation system of a steam plant, and more particularly it is concerned with a method of controlling a condensation system suitable for use with a daily start-stop plant.
A combined plant including in combination a gas turbine and a steam turbine is suitable as a daily start-stop plant, because this type of plant has good start-up characteristics due to the facts that the gas turbine lends itself to quick start-up and the steam turbine is in low-pressure and low-temperature steam condition (about 50-80 kg/cm.sup.2 g and 450.degree.-500.degree. C.). Also, in some applications, a steam plant generally referred to as an intermediate load fossil-fuel plant, which is not a combined plant, is provided, with a view to compensating for the difference in the need for electricity between daytime and nighttime.
It is well known that when the boiler feed water is high in oxygen content, the boiler tubes of a steam plant tend to be quickly corroded. To avoid this trouble, it has hitherto been customary to mount a deaerator, such as the one described in U.S. Pat. No. 3,820,333, in the condensation system, so that the condensate supplied to the deaerator during operation of the plant at load can be heated by the steam extracted from the steam turbine and deaerated, to reduce the oxygen content of feed water to the boiler to a level below 7 ppm, for example.
Also, to minimize the oxygen content of boiler feed water at plant start-up, it is practiced to seal in the plant deaerated water that has been deaerated by the residual steam in the boiler drum during operation of the plant at load when the plant is shut down for a day or two, and to use steam from some other unit or an auxiliary boiler to heat and deaerate the condensate flowing into the deaerator at plant start-up until the steam generated by its own boiler can be used for heating and deaerating the condensate flowing into the deaerator.
In the case of a daily start-stop plant, however, all the units of the power generating plant in one locality are shut down in many cases, and the auxiliary steam is unable to be readily available from some other unit. The use of an auxiliary boiler entails an increase in capital investments and has the disadvantage that the auxiliary boiler must be run every day before plant start-up.